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Forms of Corrosion (Pitting Corrosion (Condition for Pitting (Presence of…
Forms of Corrosion
Pitting Corrosion
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Condition for Pitting
Breaks of passivation film or other defects such as lack of homogeneity in passive film on metal surface
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Mechanism
- Localized adsorption of aggressive ions
- Passive film breakdown (due to corrosion potential increases ≥ Epitt
- Pitt growth (if repassivation cannot occur)
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Galvanic Corrosion
Occurs at
Junction between two dissimilar metals/ alloys and other conducting materials in the presence of electrolyte
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Factors
Environment effect
surrounding moisture, soil
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Crevice Corrosion
Caused by
Deposition of dirt, dust and mud on a metallic surface
Existence of voids, gaps and cavities between adjoining surfaces
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Mecahnism
- Anodic and cathodic reaction occur uniformly over the entire surface of metals including the interior of the crevice
- After a short interval, oxygen within the crevice is depleted due to convection restriction
- Excess positive charge is produced inside the crevice as dissolution of metal (Fe) inside crevice continues.
- Result in migration of chloride ions into crevice to balance charge
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- Both chloride and hydrogen ions accelerate the dissolution rates of most metals and alloys
- Rapid accelerate corrosion of steel inside crevice and the process is continued as dissolution rate of metal increased
- As corrosion within crevice increases, the rate of oxygen reduction on adjacent surfaces also increases
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